Identical scene, in two identical computers huge difference in render time!

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  11 November 2013
Identical scene, in two identical computers huge difference in render time!


I am new to this forum, although i am following the work posted and the tutorials here for quite some time, and i have to say that its a great effort and it has helped me on many occasions.

I have a recent problem with one machine at the office were i work.

I am an architect and i work at an architectural firm, i am the visual artist of the office and i have quite some years of experiense. I mainly use 4 of our computers which are quite high-end, with distributed rendering. Two of them are actually almost identical in specs.

The computrer i am using is an Intel core i7 -3820 CPU @ 3.60 GHz
with 40GB of RAM and a graphics card of Nvidia GeForce GTX 650 2 Gb

The computer that i call as almost identical is the same only instead of 40 GB of RAM
it has 24 GB.

The past few days i noticed a drop in the Windows Experiense Index, mainly the number of CPU calculations droped from 7.7 to 6. I started running some tests, tried to update drivers, update BIOS, to a reset on BIOS and i fortmated the computer. After many efforts on the matter the WEI came back to 7.7 from 6. But then i ran some render tests with these two computers. One two IDENTICAL scenes, with identical settings (checked thoroughly) the one computer is rendering the scene on 2 minutes while the other at 5.25. Now this difference is quite noticiable, if you think what will happen if i render i final project scene that would normaly need 10-15 hours the difference then would be too great.

Just to make clear, all the files are loeaded localy in each test, there is no distributed rendering involved, and the machines have exactly the same plugins loaded cause i need to have them identical to use them for distributed rendering on other occasions.

I send the computer to the manufacturer to check the CPU but they told me it works fine to all the stress tests (such as prime95). I loaded cpu-z and CPU meter and monitored the usage during rendering and it seems to clock normaly @ 3.60 ghz with 100% speed at all times.

Can someone please suggest something, cause i am running out of ideas. Thank you very much

  11 November 2013
Overheating will lead to the cpu slowing down, can be caused by a loose heatsink, dried thermal paste or dust on the heatsink

Some other software taking up the cpu. Does the machine show <5% cpu use when its left idle
Matthew O'Neill
  11 November 2013
I might try taking the hard drive from the faster computer and putting it in the slower one and try the same test.

I agree with imashination, that heat could be a factor. When you open up the machines, are all the fans spinning or operating at the same rpm? You can usually check fan rpms in the BIOS if the fans are connected to the motherboard.

Just for the hell of it, you might try running a memory test with memtest86. Errors can sometimes make the CPU recalculate data over again until it gets an expected result. Probably a long shot, but this is a strange scenario.
  11 November 2013
thanks for the reply both of you.

@imashination my first thoughts were the same as yours, so at some point i formated all the drives in the computer and just installed the required software and ran the test again. So no there were no programs running in the background, cpu loads are 1-2% when idle. Also with PC meter i have a constant temperature indicator, and it never goes higher than 52 C that is acceptable (on all 8 threads).

@sentry66 I will try the memory test and will let you know for any results.

A small detail that i didnt mention before is, that at some point, while trying different solutions, i did a restore default on BIOS and then restarted, and at that point i managed to make the whole system work perfect, it did the render on exactly the same time as the other computers, actually even a bit faster. But as soon as i did a restart then it continued to be slow again.
  11 November 2013
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